Coatings applied by physical vapor deposition (PVD) processes—typically performed in a vacuum—are widely used in various applications such as in creating barrier layers for packaging films, metalizing plastics for flexible electronics and EMI shielding purposes, depositing scratch-proof, corrosion protection or decorative layers on various raw materials, or for controlling the electrical, optical and tribological properties of components, tools and machine parts. Usually, different techniques may be capable of depositing the desired layers, but business economics favor processes which create the coatings quickly and efficiently. This means, the process must be able to generate large amounts of vapor rapidly, and to transport and deposit it to the substrate with low losses and at the right atomic scale structures needed for the given application.
Electron beams are established as a known tool for evaporating materials at highly achievable rates. For coating of large-area substrates, like plastic or metal films and sheets, extended evaporators heated by scanned high-power electron beams are available. It is well known that deposition rates ranging up to 10 μm/s can be achieved with this technology, i.e. the rates are several orders of magnitude higher than with sputter technology. Without applying additional aids, however, the layers grown at high rates are usually of poor quality.
Another drawback of conventional thermal evaporators—the fairly low utilization of the evaporant material when coating smaller substrates such as tools, engine parts or fibers—stems from the inherently divergent propagation characteristic of the vapor particles.
To address these issues, the development of a new coating technology, which is now called “Directed Vapor Deposition” (DVD) was started several years ago. The basic idea of the DVD concept is to evaporate the coating material by an electron beam and then to capture, transport and focus the vapor particles by a flowing carrier gas stream. This approach, fully described and disclosed in the U.S. Pat. No. 5,534,314 (of which is hereby incorporated by reference), combines the advantages of conventional EB evaporation (high vaporization rate, clean and uncontaminated material evaporation, easy alloy deposition by co-evaporation of the pure constituents from individual crucibles) with the advantages of known jet evaporators (high material utilization efficiency, possibility to vary adatom energy and spatial distribution of the vapor stream, natural mixing of vapor and reactive gas components).
In a number of applications, such as coating of fibers and metal foams, or formation of “zig-zag” structured thermal barrier coatings (TBC's) for jet engines, the DVD process demonstrated unique capabilities (non-line-of-sight coating, vapor utilization efficiency) beyond those known from established PVD technologies. However, it was also found in the course of investigations that DVD at this stage was restricted to deposition of porous or columnar microstructures. As in conventional EB-PVD, this is caused by the limited kinetic energy of the thermally generated vapor atoms. In the case of TBC's, a columnar structure is desired by the engineering purpose. For other applications or also for certain layers in the multilayer systems required in turbine blade coating, however, dense structures are demanded.
Extensive development work previously done in conventional PVD has shown that this goal can be achieved by combining the thermal evaporation process with a plasma activation of the vapor. The plasma facilitates that a remarkable fraction of the neutral vapor particles will get ionized. The ions can then be accelerated towards the substrate by the electrical fields within the plasma sheath between the bulk plasma and the substrate's surface. These fields are generally caused by the intrinsic self-bias potential of the plasma but may also be enforced by an external bias voltage. The enhanced kinetic energy of condensing particles results in densification and improved adherence of the deposited layers. By changing the plasma density, a wide range of layer modifications can be created. Further, the plasma promotes the chemical activity of reactive gases involved in deposition of compounds.
Calculations and experiments have revealed that only arc sources deliver plasma, which is sufficiently dense and capable of efficiently ionizing the vapor flux prevalent in high-rate coating. For instance, an apparatus for plasma-assisted high-rate coating has been described in the U.S. Pat. No. 5,635,087 (of which is hereby incorporated by reference). It combines electron beam evaporation with a plasma activation utilizing a transverse hollow cathode arc discharge. The process appeared to be well suited even for reactive deposition of insulating layers (oxides, nitrides) onto cold plastic substrates.
This approach has been adopted for creating a plasma-activation tool for the DVD process, too. Details of this innovation have been fully described and disclosed in the U.S. Pat. No. 7,014,889 (of which is hereby incorporated by reference). The plasma-activated DVD process has proven to be capable of high-efficient deposition and precise control of deposited coatings' composition and morphology in a great variety of applications including coatings of aircraft engine components and semiconductor wafers, among other items. In aircraft applications, coatings can be applied for both thermal and environmental barriers, as well as oxidation and hot corrosion mitigation coatings. Directed vapor deposition methods are also used to apply titanium alloy coatings to silicon carbide monofilaments to make titanium matrix composites, and to infiltrate silicon carbide fiber performs with SiC to make (SiC/SiC) ceramic matrix composites. The use of plasmas also greatly enhances vapor phase reaction rates enabling the synthesis of hard materials such as titanium carbide and various nitrides.
The conventional plasma assisted deposition process has several drawbacks, however. First, the plasma source's working gas emitted from the hollow cathode forms a high speed jet whose axis is at right angles to the direction of vapor transport. Slow moving or light (i.e. low momentum) vapor particles can be scattered away from the substrate by the working gas jet of the hollow cathode. Second, the conventional approach requires the use of high argon working gas flow rates which has adverse economic consequences. It also requires a more powerful vapor transporting gas jet which has economic consequences because of the greater use of the helium gas and need for higher capacity pumping systems. Third, there is no means for sweeping the vapor plume from side to side (i.e. paint spraying a large area surface) in the conventional arrangement without significantly effecting the plasma properties. Fourth, the conventional plasma generation approach provides inadequate cleaning, etching, and heating properties for some applications (i.e. the deposition of high temperature materials onto large area substrates).